1. Field of the Invention
This invention relates to the field of plant nutrients, particularly those metal compounds classified as micronutrients, and more particularly to a method for eliminating dusts from granules of metal sulfate micronutrients and from the processes producing and handling these granules.
2. Description of Prior Art
Micronutrients are used alone, or in combination with primary and secondary nutrients, as plant foods. They may be in the form of liquids, powders, or granules. Granules are desirable because they may be conveniently used in simple solids handling equipment, and the metal sulfate granules are particularly desirable because of their ease of production, relatively low cost, high water solubility, and ready availability for absorption into growing plants. The metals most commonly produced as sulfate micronutrient granules are: zinc, manganese, iron, and copper. The sulfates produced depend upon the type granulation process used and the amount of product solubility desired.
Granules of micronutrient sulfates are usually formed by blending sulfuric acid with finely divided metal oxides in a rotating drum to form a wide variety of granule sizes. The desired granule sizes, normally 6 to 16 U.S. Mesh, are recovered by screening, and the larger-sized granules are ground and recycled to the rotating drum as powder, along with the undersized particles. A tilted, rotating pan, or a fluidized bed apparatus, may be used instead of a rotating drum to form the metal sulfates.
To produce high purity metal sulfate granules, metals, or metal oxides are dissolved in sulfuric acid and water, where feasible, and any insoluble impurities are removed by filtration. Granules are produced by spraying the purified solution onto a bed of recycled metal sulfate solids and the resulting granules are dried by application of heat.
Dust is created in the metal sulfate processes: by physical attrition of the moving granules; by fine particles of raw material which, whether reacted to sulfates or not, do not agglomerate to granules; and by grinding recycled metal sulfate oversize granules. Dust may also be created by the addition of other dry powders to the granules. Dust may also be caused, in the absence of any chemical process, by physical abrasion which occurs in conveying, loading, unloading, or storing procedures.
Dusts on, and around, granules of micronutrient sulfates frequently cause the granules to stick together in lumps, to handle poorly, and to develop a poor condition during periods of storage which makes the granules difficult to handle and particularly troublesome to apply. Dusts of metal sulfate around micronutrient production, or handling, processes constitute an economic loss and, more importantly, may be hazardous to the workers in the area. Although lead is not considered a micronutrient metal, it is present in several sources of the micronutrient metals converted into sulfate granules, particularly zinc. Thus, it is important not only to recover all of the micronutrient metal sulfates present as dusts but also to remove all the lead dust which has an established toxicity level.
The micronutrient and fertilizer art shows that little work has been done to overcome the two problems of economic losses and health hazards from unrecovered dusts from micronutrient granules of metal sulfates. Some work has been reported on containing micronutrient metals as particles having high structural integrity. Tisdale and Nelson describe several such techniques in Soil Fertility and Fertilizers, The Macmillan Company, New York, 1966. On page 342, they state that zinc can be enveloped in frits. On page 395, they state that the addition of oil helps micronutrient powders to ahdere to fertilizer granules. On page 392, they describe developments which allow encapsulation of granular fertilizers to produce water impermeable coatings, giving a metered supply of nutrients and improving fertilizer condition. Various coating substances, such as plastics, resins, waxes, paraffins, elemental sulfur, and asphaltic compounds were cited as being under investigation.
Beat Meyer in Urea-Formaldehyde Resins, Addison-Wesley Publishing Company, 1979, Reading, Mass. told of the use of formaldehyde by Karnemaat to produce a dust-free N-P fertilizer. In U.S. Pat. No. 4,033,745 Moore disclosed a storable methylol-methylene solution which has the potential of polymerizing to water insoluble solids. In U.S. Pat. No. 3,991,225 Blouin discloses a method for sulfur coating fertilizer granules, particularly urea.
The art defines methods for coating fertilizers, for controlling nutrient release rates, for improving fertilizer condition, and for encapsulating granular fertilizers. The art also discloses several fertilizer coating materials. No mention was found in the art of a method for collecting and occluding dusts in a monomeric liquid, coated on the surface of metal sulfate granules, which metal sulfates catalyze the polymerization of the monomeric liquid through an adhesive stage to a hard film coating occluding and containing the dust particles. The art provides on method for improving the safety and increasing the recoveries in method sulfate granulation processes, nor does it provide operating parameters from which such a method might be resonably derived.